Precipitation strengthened alloys

ABSTRACT

A precipitation strengthened alloy is provided having the composition: Tantalum 5-20 Tungsten 2 Chromium up to 30 Iron0-10 Carbon 0-0.3 Nickel 0-30 Silicon 0-1 Yttrium 0-0.2 Lanthanum0-0.2 Manganese 0-2 Cobalt + incidental impurities balance WHEREIN THE RATIO OF TANTALUM TO TUNGSTEN IS BETWEEN ABOUT 1 1/2 TO 2 1/2 AND THE MOLYBDENUM IS LESS THAN 1 PERCENT.

United States Patent n91 Augustine, Jr. et a1.

[ PRECIPITATION STRENGTHENED ALLOYS Inventors: Coleman M. Augustine, Jr.; Robert B. Herchenroeder, both of Kokomo, 1nd.

Assignee: Cabot Corporation, Kokomo, Ind.

Filed: Jan. 12, 1973 Appl. No.: 323,101

u.s. Cl 148/325, 75/170,

[56] I References Cited UNITED STATES PATENTS 3,366,478 l/l968 Wheaton 75/171 m m w 5 [111' 3,802,934 [451 .Apr. 9, '1974 Primary ExaminerRichard 0. Dean Blenko having the composition:

Tantalum 5-20 Tungsten... 2-15 Chromium. up to 30 Iron .1. ..0-l0 Carbon ..0().3 Nickel ..0-30 Silicon .,.01 Yttrium 0-0.2

. Lanthanum 0.2 Manganese... 2 Cobalt+incidental impurities... .balan ce wherein the ratio of tantalum to tungsten is between about 1 /2 to 2 /2 and the molybdenum is less than 1 V percent.

6 Claims, 1 Drawing Figure 1 PRECIPITATION STRENGTHENED ALLOYS This invention relates to precipitation strengthened alloys and particularly to precipitation strengthened cobalt-base alloys.

While good cobalt-base alloys exist for high temperature service (for example, HAYNES alloy No. 25 and HAYNES alloy No. 188), these alloys are characterized by solid solution strengthening as contrasted to strengthening by the formation of precipitates (for example, 7 precipitation in nickel-base alloys). Those cobalt-base alloys that have been strengthened by precipitation reactions have generally employed aluminum and/or titanium as reacting elements and these alloys are characterized by improved strength near 1,200F, but ineffective strengthening at 1,600F or above.

Others have employed beryllium, columbium (niobium) or tantalum. These elements will cause precipitation reactions in cobalt alloys, but heretofore the strengthening mechanism(s) have not been effective at l,600F and above. a

We have found that cobalt-base alloys having improved strength at 1,600F and higher can be achieved by what appears to be a precipitation reaction caused by the presence of tantalum to tungsten in the ratio of about 1% to 2 /2 and preferably in the ratio of 2 to .1 by weight percent and the molybdenum is limited to impurity levels and the carbon is maintained below about 0.3 percent. I

It is an object of this invention to provide cobalt-base alloys with improved strength at 1,600F and higher.

Another object is to provide a high strength wrought cobalt-base alloy.

Yet another object is to provide a high strength cobalt-base alloy which is oxidation resistant.

Other objectives are to provide cobalt-base'alloys that are formable and which retain' good engineering strengths at temperatures as high as 2,000F.

Still other objects will be apparent from the following description and claims. Q

A cobalt-base alloy in accordance with the present invention is broadly an alloy consisting essentially of about: 4

wherein the ratio of tantalum to tungsten in weight percent is between about l'r to 2% and the molybdenum is less than 1 percent.

A preferred range of the alloy providing useful strengths is an alloy consisting essentially of about:

Weight Tantalum 5-20 Tungsten 2-1 5 Chromium l5-30 9 2 /2 and the molybdenum content isless than one perwherein the ratio of the tantalum to tungsten in weight percent is between about l /to 2 /2 and the molybdenum is less than one percent. A more preferred range of the alloy is:

Weight Tantalum 5-18 Tungsten 2-12 Chromium 15-30 Iron 0-10 Carbon 0-0.2 Nickel 8-30 Silicon 0-l Lanthanum 0-0.2 Yttrium v 0-0.2 Manganese 0-2 Cobalt incidental impurities balance Wherein the ratio 'of the tantalum content to the tungsten content in weight percent is between about 1% to cent.

In addition to the specifically mentioned constituents, other alloying elements may be added without departing from the spirit of the invention arid without negating the criticality of the tantalum to tungsten ratio and the minimization of the molybdenum and carbon contents. Such elements would include, but would not necessarily be limitedto, Hf to about 5 w/o, Ti and/0r Zr to about 2 w/o, Cb (Nb) and Re to about 4 w/o, aluminum to about 1 'w/o, and magnesium and/or boron to about 0.04 w/o. v

It has been discovered, as part of the present invention, that alloys as described above develop unusual strengths for a' cobalt-base alloy through at least 1,700F, when the tantalum contentof the alloy is approximately twice that of the tungsten content and when the carbon is maintained at a relatively low level. When the critical Ta/W ratio is not adhered to, effective strengthening is not obtained.

The strength'sdeveloped apparently are caused by a precipitation reaction of some sort because, as will be shown, in the annealed condition (heat treated about 2,2009F and rapid cooled) the materials are relatively achieved are significantly less. Therefore, molybdenum -is considered as a detrimental impurity in the alloy and is only tolerated to l w/o for economic reasons.

Apparently, carbon also inhibits effective strengthening by the mechanism and therefore is limited to a maximum of 0.3 weight percent. Carbon in excess of this amount drastically reduces the strength achieved.

it is anticipated but not ,proven that iron and nickel 10 would function as a partial participant in the strengthening mechanism.

I Table l hereinbelow lists the compositions of alloy used to demonstrate the invention.

and then rolled into sheet, annealed between 2,l50 and 2,200F, and rapid cooled.

Alloy 24 was vacuum induction melted and then cast into test specimen mold prepared per the loss wax. process. i 7

Alloy 9 is considered an example of the invention, while alloy 10 demonstrates the detrimental effects of molybdenum, alloy 7 demonstrates that tantalum without tungsten is not effective, alloy demonstrates that near equal amounts of tantalum and tungsten are not effective, and alloy 24 illustrates that excessive amounts of carbon are to be avoided.

Table 11 hereinbelow lists comparative tensile TABLE I CHEMICAL ANALYSlS. w/Q

Alloy Al C Co* c1 Fe La Mn Mo Ni Si Tu W 5 0.33 0.1 Bal 21.56 1.88 0.05 0.66 0.37 23.60 0.35 7.81 9.31 7 0.38 0.12 Bal 21.07 1.86 0.04 0.67 0.33 23.60 0.40 16.74 9 0.37 0.12 3:11 20.81 1.36 0.05 0.53 0.42 23.50- 0.27 10.53' 4.50 0.45 0.12 B211 20.07 1.38 0.04 0.54 4.04 22.40 0.25 10.14 4.39 24 0.19 0.55 Bal 20.88 1.18 0.07 21.63 10.08 4.95

' Cobalt plus incidental impurities No W added 10 melt In general, the alloys of Table ,1 were vacuum induction melted (although other methods might have been used), cast into nominally 20,-pound roundtapered ingots, forged from a furnace operating about.2,lF

TABLE 11 TENSlLE DATA* Test Temp. YS UTS. Elongation A116 1= ksi ksi 5 Room 75.7 1417.7 49 Room 77.4 146.5 49

7 R661 x4 7 152.9 32 R6616: 80.5 154.2 7 32 9 ROUITI 55.0 133.6 63- Room 54.6 133.2 60- Annealed ROOm 132.7 1113.0 20 +16 lll'S.

TABLE II Continued TENSILE DATA* Annealed unless otherwise noted.

The data of TABLE II show'that alloys 7, 9, and

all have improved intermediate temperature strength as compared to commercial alloy 188, which for comparison has tensile properties at 1,600F of about 38 ksi 0.2 percent offset yield strength, 61 ksi ultimate strength and 69 percent elongation. The tensiledata for. alloy 5 illustrate that when the critical Ta/W ratio of about 2 is not adhered to, the strengthening of the alloy is minimum. Alloy 5 has 7.81 w/o Ta and 9.31 w/o W.

It is worthy of note that alloys-9 and l0 had higher yield strength at 1,600F than at room temperature. This is' thought to be the result of a precipitation-reaction occurring during the to 60 minute stabilization period at temperature prior to tensile testing. Further proof that a precipitation react-ion occurs in the alloy of the invention is that alloy9 aged 16 hours at l,500F

' had more than double the yield strength at room te m perature than did the alloy without an aging treatment.

Alloys similarto the alloy of the invention (alloy 24) but with carbon levels exceeding about 0.3 weight percent apparently are not strengthened in the same manner as the alloys of the invention.

For example, alloy 24 was aged 24 hours at 1,800F then tested at room temperature with no significant increase in strength being observed over a similar test bar hereinbelow show the stress rupture properties of the alloys. These data and the graph'of the FIGURE 'show clearly the stress-rupture'life properties.

TABLE lll srmzss RUPTURE DATA Test Temp. Stress Life Elongation Alloy F ksi hrs.

1500 4.1 9 Annealed l6 hrs. at l500F 9 1500 25 654.0 11 1500 25 626.2 s 1500 40 47.6 28 Annealed 16 I hrs. at l500F tially in weight percent of about:

TA BLE Ill Continued STRESS RUPTURE DATA I i Test Temp. Stress Life Elongation Alloy k'si hrs. 10 1500 '25 229.3 8 1500 25 231.8 6

Samples annealed 0.05" thick sheet unless noted otherwise.

A comparison of the data from alloys 9 and 10 illusa i s :5 2

. 1 at n trates a good example of the detrimental effects of mo- Nickel L30 lybdenum on the strength of the alloys of the invention. Cobalt incidental impurities balance Alloys 9 and 10 were produced from the same'meltof material, forged atthe sametime, rolled at the same time and otherwise processed identically. The only significant difference being that, after the ingots for alloy 9 were cast from the melt, a late addition of molybdenum was made to modify the composition. A lanthanum addition of 18 gms and 30 gms of a 20 Mg 80 Ni alloy were also added-to replace the losses of lanthanum and magnesium which occurred during the melt-in of the molybdenum late addition.

. The data from alloys 9 and 10 show that alloy 10 containing 4.04 w/o molybdenum has approximately one half the rupture life of alloy 9 containing only an impurity level of 0.42 w/o molybdenum.

' The foregoing data clearly show that a superior cobalt-base alloy is provided by this invention having a critical relationship between the tantalum content and the tungsten content and wherein thernolybdenum and carbon contents must be critically controlled.

It will be apparent to men skilled. in this art that, while we haveillustrated and described certain preferred embodiments of this invention in the foregoing specification, the invention may be otherwise embod ied within the scope of the following claims.

1., 1A precipitation hardened alloy consisting essen- Tantalum 5-20 Tungsten 2-l5 Chromium v up to 30 iron 0-10 Carbon 0-0.3 Nickel 0-30 Silicon 0-1 Yttrium 0-0.2 Lant h'anum 0-0.2 Manganese, 0-2 v Cobalt incidental impurities balance 1 l wherein the ratio of tantalum to'tungsten is between I about 1% to 2% and the molybdenum is less than one percent.

2. An alloy as claimed in claim 1 consisting essenwherein the ratio of the tantalum content to the tungsten content is about 1 /2 to 2% and the-molybdenum content is less than 1 percent.

' 3. An alloy as claimed in'claim l consisting essentially in weight percent of about:

Tantalum 5-18 Tungsten 2-12 Chromium l 5-30 lron 0-!0 Carbon 0-0.2' Nickel 8-30 Silicon O-l Lantlianum 0-0.2 Yttrium I 0-0.2 Manganese 0-2 Cobalt incidental impurities balance 4. An'alloy as claimed in claim 1 consisting essentially in weight percent of about:

Tantalum I Tungsten 2-8 Chromium 15-30 lron 0-10 Carbon 0-0.2

Nickel 15-30 Silicon 0-1 4 Lanthanum 0005-02 Manganese 0-2 I Cobalt incidental impurities balance wherein the ratio of the tantalum to tungsten content is about 1% to 2% and the molybdenum content is less than 1 percent.

5. An alloy as claimed in claim 1 consisting essentially in weight percent .of about:

Tantalum 8-l2 Tungsten 3-6 Chromium 20-24 iron .0-2 Carbon 0-0.]5 Nickel ,l.525 Silicon 0-0I5 Lanthanum 0.005-0.l5 Manganese 0-l Cobalt incidental impurities I balance 

2. An alloy as claimed in claim 1 consisting essentially in weight percent of about:
 3. An alloy as claimed in claim 1 consisting essentially in weight percent of about:
 4. An alloy as claimed in claim 1 consisting essentially in weight percent of about:
 5. An alloy as claimed in claim 1 consisting essentially in weight percent of about:
 6. An alloy as claimed in claim 1 wherein the ratio of tantalum to tungsten is about 2 to
 1. 